They predicted that another eruption on the southern island would bury 7million underneath lava flows, while westerly winds would carry a huge pall of ash and dust to Honshu.
The toxic cloud would make the neighbouring island 'unliveable', they said, while saving the 120million living in major cities and towns would be 'hopeless'.
Professors Yoshiyuki Tatsumi and Keiko Suzuki calculated that the risk of such an eruption occurring in the next 100 years was around 1 per cent.
However, they said that figure could not be ignored, as the chance of a major earthquake striking Kobe within 30 years was estimated at about one percent just a day before a 7.2-magnitude quake destroyed the Japanese port city in 1995, killing 6,400 people.
'Therefore, it would be no surprise if a colossal eruption occurs at any moment,' the study added.
The news comes just weeks after Mount Ontake, in central Japan, erupted suddenly in September, killing 51 people, some of whom were children.
Most of those who died were hikers who were standing and taking pictures near the summit when the mountain erupted in the country's worst volcanic disaster for 90 years.
It also comes amid warnings that a volcano in southern Japan located 40 miles from a nuclear plant was showing signs of increased activity that could possibly lead to an eruption.
Ioyama, a mountain on the south western island of Kyushu, has also been shaken by small tremors and other signs of rising volcanic activity recently, indicating that it might also erupt.
One tremor lasted as long as seven minutes, an official at the Japan Meteorological Agency's volcano division said.
- Daily Mail
